The invention relates to a method for autofocusing of a microscope having continuously or discontinuously variable objective magnification. It furthermore relates to a microscope having continuously or discontinuously variable objective magnification.
During microscopic investigations, it is often necessary to be able to change between different magnifications quickly. Objectives are used whose magnification varies over wide ranges, for example from 5.times. to 150.times. and beyond. Since such microscopes are also used to a large extent for monitoring in production, for example for routine investigations during wafer production, and since the visual focusing procedure is extremely tiring for the operator, attempts are being made to an increasing extent to bring autofocusing systems into use which permit quick automatic definition setting onto the object, which in individual cases is also more precise than would be possible visually.
However, it has been found that conventional autofocusing systems are overloaded when the objective magnification varies in a relatively large region, as indicated, for example, above. This overload is especially pronounced if the system is operating with radiation which prevents damage or a change in the semiconductor wafers to be observed. The overload is on the one hand caused by the fact that, in the case of increasing magnification, structures which are contained in the object or are generated on the object are smeared with respect to the edge contrast. This leads to increasing inaccuracy when this contrast, or variables derived from it, such as the high location frequencies content, are used for definition setting. A further problem noted by the applicant is that the depth of field on the image side increases with increasing magnification, with the square of the magnification of the objective to be precise. This may contribute to a considerable extent to the partially unexplained failure of conventional autofocusing systems in the case of differing objective magnifications.
DE-OS 3,707,487 discloses a method for autofocusing and a microscope having an autofocusing system which are capable of permitting reliable and optimum automatic definition setting onto the object even in the case of widely differing objective magnifications. However, it has been found that this type of autofocusing works completely only when sufficient details are visible on the object. Especially in the case of the direct illumination method, it is often necessary to focus on object points having few details--for example wafers in a first coating stage--or, in the extreme case, even on a pure mirror surface.
However, even when sufficient details are visible in the case of rapid movement of the object table with a simultaneously powerful objective magnification, the image may move so rapidly through the optimum definition setting that the electronics can no longer recognize it as being clear or unclear.
Depending on the selected objective magnification (aperture) and on the magnification changer stage in the stand of the known microscope, the definition signal curve shown in FIG. 3 of DE-OS 3,707,487, when going through focus, has a different shape. If the two definition signals are too far from one another, an undefined region exists between the two signals and the autofocus stops somewhere within this region.
On the other hand, the situation can arise that the two definition signal regions overlap too much, so that the resulting signal difference is too small. In the case of a conventional autofocusing system, these said difficulties were overcome by matching the definition signal curves to the respective objective/magnification changer combination by means of a zoom system in the autofocusing beam path to the fixed distance of the two TV targets.
DE-OS 3,739,223 describes a further development of the autofocusing method and system disclosed in DE-OS 3,707,487 such that reliable definition setting is ensured, especially during direct illumination operation, even in the case of objects which themselves only have few details. A pattern containing light-dark contrasts is projected into the object plane. The pattern which is projected onto the object generates an image there which is reflected and is used for definition setting. In this case, the term "signals" comprises both optical and electrical signals. Optical signals occur first in the autofocusing system, and are converted by electro-optical converters into electrical signals which can be subjected to further electronic processing and, after their processing, supply at least one control signal which can cause a displacement of the object and/or objective in the direction of the optimum definition setting, in a manner known per se, for example via suitable electric motors and drives.
However, it has been found to be disadvantageous for both known autofocusing systems to provide a zoom system in the autofocusing beam path, since it varies the magnification of both partial images in such a manner that, in an extreme case, the latter are much larger than the targets, which leads to loss of light intensity or, in another extreme case, are much smaller than the targets, which leads to an adverse effect on signal evaluation as a result of the remaining dark edge region.